This invention relates to a thread tension device for sewing machines such as an overlock sewing machine and the like. More particularly, the invention relates to a thread tension regulating mechanism which is capable of regulating or controlling energizeable force applied to threads in response to changes in the shapes of stitches to be formed so that the threads may be suitably adjusted for sewing operations.
FIG. 4 shows a perspective view of a conventional overlock sewing machine provided with a plurality of spool holders 1 (four spool holders in this instance). The spool holders are arranged to unwind the respective threads in such a manner that they intersect each other to form an overedge stitch, a roll hemming stitch, and an ornamental stitch or the like.
Generally, the overlock sewing machine varies tension on a plurality of the threads through resistance to movement whenever changes in the shape of stitches are made. In order to correspond to such changes in tension on the threads, the overlock sewing machine is normally provided with thread tension devices 2, each engaging each thread and applying a tensile force thereto. FIG. 5 is a sectional view showing the details of the thread tension device. For the purpose of illustration, only one thread tension device 2 is shown, although four thread tension devices are actually mounted as to correspond to four threads. A thread tension dial 3 is mounted on the frame of the sewing machine to rotate a shaft 4. A pair of thread tension discs 5 are so supported on the shaft as to be slidably movable axially on the shaft 4 but are prevented by a stationary member 10 fixed to the shaft 4 from being moved in a direction opposite thread tension dial 3 because the thread tension discs 5 abut against the stationary member 10. A slider member 6 is also supported on the shaft so as to be slidably movable axially on the shaft 4 and is provided at one end thereof with a pin P. A cam 8 is secured to the shaft 4 and rotated therewith. A cam groove 9 is obliquely formed in the cam 8 to receive the pin P of the slider member 6.
A spring B is mounted over the shaft 4 and adapted to abut its one end against the thread tension discs 5 and the other end against the slider member 6. Although only one thread tension device is shown for the purpose of better illustration, a plurality of the thread tension devices may be mounted on the shaft 4 and provided with corresponding cams 8 having cam grooves 9 which are configured different from each other to apply the optimum tensile force to the tension devices and to the threads.
According to the thread tension device arranged as aforesaid, rotation of the thread tension dial 3 rotates the cam 8 causing the cam groove 9 to axially move the slider member 6. Upon movement of the slider member 6, the spring B is caused to vary energy force to the thread tension discs 5, thereby varying tension through resistance to movement of the thread clamped between the tension discs.
The conventional tension regulating device has the following problems. The thread tension device for the overlock sewing machine shown in FIG. 5 is arranged so as to be movable by a pin P outwardly extended from the slider 6 and a cam groove 9 in which the pin P is received so that a contact surface 7 defined by the slider 6 and the shaft is elongated. In other words, if the contact surface is narrowed, the slider member 6 is tilted when moved to thus prevent it from being smoothly moved. For this reason, the thread tension device shown in FIG. 5 is designed so that the slider 6 is made longitudinally and axially of the shaft 4 and is thus elongated as a whole. Thus, the overlock sewing machine is inevitably made wider as seen from FIG. 4 and has resulted in the massive sewing machine. Furthermore, great force is required to move a plurality of the sliders 6 so that manipulating the thread tension device is difficult, particularly, rotating the thread tension dial 3 by the pin P extending therefrom and the cam groove 9. This requires the shaft 4 to provide an axially longitudinal contact surface to have the slider member 6 contacted therewith.